This invention relates to a process for the decolorization of pulp mill bleach plant effluent.
The effluent from a pulp mill bleach plant contains highly colored lignin degradation products and is thus itself densely colored. Since pulp mill bleach plants produce very large quantities of this densely colored effluent (typically several thousand liters of such effluents are produced per ton of bleached paper product produced), the discharge of such large quantities of densely colored effluent into running water causes a highly objectionable discoloration of the water. Already-existing or future pollution control regulations will force pulp mill bleach plants to reduce the color in their effluent in the near future.
The lignin degradation products responsible for the color of the effluent are partially aromatic, polymeric materials very resistant to biological degradation. Although it is now known that certain bacteria can degrade lignin slowly, only the higher fungi can effect rapid degradation. Traditional biological waste treatement processes, which do not use higher fungi, are ineffective in removing color from pulp mill bleach plant effluents.
Conventional non-biological effluent treatment processes such as ultra-filtration, reverse osmosis, precipitation with lime or alum and carbon absorption are effective in removing color from such effluents, but are prohibitively expensive for use in treating the quantities of such effluents generated by commercial-sized plants because of the need for large amounts of chemical reagents, high maintainance costs and expensive facilities. To reduce these costs and to improve the effectiveness of chemical precipitation, it has been suggested that ferric chloride and lime be used together in such a precipitation method; see Dugal, Church, Leakley and Swanson, Color Removal in A Ferric Chloride-Lime System, Technical Association of Pulp and Paper Industry 59(9), 71(1976).
It has also been suggested that the effluent be decolorized by treating it with lignite coal fly ash acidified with chlorination stage effluent to solublize aluminum ions therefrom; see Bakhshi and MacDonald, Colour Removal from Pulp Mill Effluents Using Fly Ash-Mini Pilot Plant Experience, AICHE Symposium Series, Vol. 76, Lightsey (ed.), American Institute of Chemical Engineers, N.Y., (1980). This method only removes color when operated between pH 4.0 and pH 4.6 and has the serious disadvantage of requiring large quantities of the fly ash; although such fly ash, of course, readily available at coal-burning power plants, many pulp mills are situated in rather remote areas far distant from any such power plants and the transportation of fly ash to the pulp mills renders the method economically unattractive.
It is known that precipitation of the color from the effluent can be effected by lowering the pH of the effluent below about 1.5, see Sameshima and Kondo, Study of the Color of Waste Liquor of Pulp Industry. I. The Relationship Between the Color of Waste Liquor in the Kraft Pulp Multistage Bleaching and the Isolated Cl.sub.2 -oxylignin, Mokuzai Gakkaishi 16, 347 (1970), and U.S. Pat. No. 4,000,033 to Nicolle et al. This method has the disadvantage of requiring large quantities of acid and of producing a very acidic treated effluent, the disposal of which may be as difficult, if not more difficult, than that of the original colored effluent.
There is thus a need for a process for the decolorization of pulp mill bleach plant effluent which does not use large quantities of expensive chemical reagents, does not involve high transportation costs and which does not need extensive plant or extensive maintainance of such plant. This invention seeks to provide such a process.